Naphtha sweetening with a phenylenediamine followed by alkali



Get. 9, 1956 1 v". soRG v NAPHTHA SWEETENING WITH A PHENYLENEDIAMINE kFOLLOWEID BY ALKALI Filed Aug. 25, 1953 ATTWE Unite States Patent NAPHTHA Sl/VEETENING WITH A PHENYLEN'E- DIAMINE FLLOWED BY ALKALI Leonard V. Sorg, Kansas City, Mo., assigner to Standard Oil Company, Chicago, Ill., a corporation of Indiana Application August 1953, Seriai No. 376,298

3 Claims. (Cl. `196--29) This invention relates to the rening of cracked, sour naphthas. Particularly the invention relatesl to the sweeteni-ng of cracked, sou-r raw-naphthas, i. e.,` those Which have not received any form of treatment subscquent to the production thereof by a cracking process.

The sweetening, i. e., elimination of mercaptans, of cracked, sour naphthas is rendered dicult by the presence of reactive substituents, particularly gum-forming bodies. Copper chloride sweetening of cracked', sour naphthas resul-ts in the production of Ia sweet naphtha that is quite unstable with respect to color; the addition of Vlarge amounts of metal deactivator is necessary to produce a stock of satisfactory stability. Doctor sweetening is frequently unsatisfactory because the sweet naphtha contains corrosive sulfur.

Recently a new process has come into wide use for the `sit'eetening of cracked, sour naphthas. ThisV process is now commonly known `as the inhibitor-sweetening method. in this process oxidation inhibitors of the phen-yiene diamine and alkylphenol types are added to the sour n-aphtha in the presence of aqueous caustic solution and free-oxygen. Various modiiications of this technique have appeared in order -to increase the rate of sweetening which normally is very slow. One of the better known methods comprises treating the sour, cracked naphtha with aqueous caustic solution to remove H28 and `some mercaptana the addition of the inhibitor and free-oxygen to the extracted naphtha, subsequently contacting this mixture with aqueous caustic solution and separating a naphtha substantially reduced in mercaptan content from aqueous caustic solution. if the naphtha is not substantially sweet at this point, normal- 1y, it will become sweet after being in storage for a day Vor two.

While the ,inhibitor `sweeteuing process is .etective .and does-not suffer from the disabilities of the .other conventional processes, it has been found to produce sweet naphthas that contain `excessively high gum contents. This gum content which is developed instorage over a period of several months can be predicted by means of the accelerated gum test.

It is an object of Vthis invention to sweeten cracked, sour napthas. Another ,object of the invention is to sweeten cracked, sour naphthas by the inhibitor method. Still another object is an inhibitor-sweetened naphtha which has .satisfactory gum stability. A particular object is the inhibitor sweetening of catalytically cracked naphtha to produce a substantially sweet naphtha of satisfactory gum stability. Other objects of the invention will become apparent in the course of the detailed description thereof.

it -has been found thatthe gum stability of an inhibitor-sweetened cracked naphtha can be improved by a process which comprises adding to a cracked, `sour rawnaphtha between about 2 and 20 pounds of a phenylene diamine inhibitor per 1000 vbarrels (42 gal.) of said naphtha, contacting said mixture with at least enough aqueous caustic solution to form a distinct aqueous phase,

in the presence of free-oxygen in an amount about the stoichiometric equivalent of mercaptan sulfur in said naphtha for a time suiicient to substantially reduce the mercaptan content of said naphtha. A substantially sweet product naphtha is separated from the aqueous caustic phase. Preferably the contacting is carried out at a temperature between about and 100 F.

The raw naphtha charged to the process is al naphtha derived from a cracking process. The cracking process may either be thermal cracking or thermal reforming or catalytic cracking. The catalytic cracking process may be any one of the processes now conventionally used such vas fluid catalytic cracking, Houdry cracking, thermofor catalytic cracking, Houdry flow, etc, The catalyst used in the catalytic cracking process may be clay-type or synthetic catalysts such as silica alumina and silica magnesia.

The term raw-naphtha. is intended to include vnaphthas which 'have not received any chemical treatment which results in the removal of appreciable amounts of their ordinary constituents, particularly the removal of oxygenated compounds. The raW-naphtha of this invention may be either the naphtha. as produced from the fractionation ofthe liquid product of the cracking of Ygas oil or heavy naphtha; or it maybe that naphtha which has been stored without any chemical treatmentv to remove HZS, cresois, etc. I-t is preferred to` use asa feed to the process` raw-naphtha directly from the fractionator in order to avoid oxidation by atmospheric oxygen while in storage.

The sweetening agent used in the process of this vinvention is one which acts as an oxidation inhibitor as well as a catalyst for the conversion of mercaptans to disuldes. This sweetening agent must be essentially insoluble in aqueous caustic solutions. The sweetening agent .comprises a phenylene diamine type inhibitor and more particularly N-,NCdi-secondary-butyl-p-phenylene diamine.V However, it is understood that other phenylene diamine inhibitors may be. employed including lN,N^- di-aikyl-p-phenylene diamine-s in Awhich the alkyl `groups contain 4from l lto about 12 carbon atoms per ymolecule including such compounds as N,N diiso -propyl p phenylene diamine, N,N'-di-amyl-p-phenylene diamine, N,`N"-di-hexyl-p-phenylene diamine, etc., as -well as those 'in which the alkyl groups are different as, for example, in such compounds as N-propyl-N-butylpphenylene diamine, N-butyl-N'-amyl-p-phenylene diamine, N-hexyl- Noctyl-pphenylene diamine, etc.

The amount of phenylene diamine inhibitor utilized in the process will vary somewhat with the -type of raw- -na-phtha ycharged and also with the operating conditions. In general the phenylene diamine inhibitor usage will be at least about 2 pounds per 1000 barrels (42 gal.) of raw-naphtha. Amounts as much as 20 pounds or more may be used in some instances. Excessive usage has no harmful effect; however, it is uneconomic. It is preferred to use lbetween about 4 and '10 pounds of phenylene diamine inhibitor per 1000 barrels (42 gal.) of rawnaphtha.

The aqueous caustic solution may contain either sodium hydroxide or potassium hydroxide. In general the aqueous caustic Isolution will contain about -10 Weight percent of caustic and the saturation amount, about' 50 weight percent. It is'preferred to use an aqueous caustic solution containing between about l5 and 30 weight percent of alkali metal hydroxide.

At least enough aqueous caustic solution must be Yused in the contacting of the raw-naphtha-inhibitor mixture to have present a distinct aqueous caustic phase. In some instances satisfactory results are obtainable by having only enough aqueous caustic solution present to form a haze in the mixture. More than this amount is usually desirable. Generally at least enough aqueous caustic solution is used to form a distinct, separate aqueous phase. Based on raw naphtha charged, the amount of aqueous caustic solution used may be between about and 100 volume percent.

The sweetening does not occur in the absence of freeoxygen. It has been found that the objects of the inven tion are attainable only if the naphtha-inhibitor mixture is contacted with aqueous caustic solution prior to or simultaneously with the addition of free-oxygen. The free-oxygen may be derived either from the atmosphere by the use of air or by means of commercial cylinder oxygen. The amount of free-oxygen needed is at least about the stoichiometric equivalent of mercaptan sulfur present in the naphtha, i. e., 1 mol of free-oxygen per 4 mols of mercaptan or 3 standard cubic feet of oxygen per pound of mercaptan sulfur. More than this amount is desirable and it is preferred to use between about 150 and 250% of the stoichiometric requirement.

The process of the invention may be utilized at temperatures as high as about 200 F. Temperatures as low as about 50 F. may be used when reaction rate is of little moment. It is preferred to operate at temperatures below about 110 F. as the naphtha sweetened at these temperatures has a more agreeable odor than does the naphtha sweetened at temperatures such as 150 F. or higher. It is preferred to carry out the sweetening step at a temperature between about 80 and 100 F.

The process may be carried out by contacting the naphtha-inhibitor mixture with aqueous caustic solution and substantially simultaneously with free-oxygen. The contacting is maintained for a time suicient to substantially sweeten the naphtha, The process may also be carried out by contacting the mixture with aqueous caustic solution, separating an aqueous caustic phase from a phase comprising naphtha and some occluded aqueous caustic solution and contacting this naphtha phase with free-oxygen until a substantially sweet naphtha is obtained. The occluded aqueous caustic solution may be separated from the sweet naphtha by a settling operation or by passage through a coalescer such as a Salt drum or sand lter. Frequently the amount of occluded solution is so slight that normal storage will result in the separation of suicient aqueous solution to permit the naphtha to be used for commercial products without any special further dehazing treatment.

The preferred method of operation of the process comprises contacting the naphtha-inhibitor mixture with an aqueous NaOH solution containing between about and 30 weight percent NaOH in an amount sufficient to form a separate aqueous caustic layer, separating the extracted naphtha from an aqueous caustic layer, adding the desired amount of free-oxygen to the extracted naph tha, contacting the extracted naphtha-oxygen mixture with aqueous NaOH solution in about the same concern tration and quantity as in the first contacting step and separating a sweet product naphtha from aqueous caustic solution. The process is carried out at a temperature between about 80 and 100 F.

'Ihe results obtainable by the process of this inven tion are illustrated below. The cracked, sour raw-naphtha feed in all cases was a stabilized heavy naphtha boiling between about 130 and 400 F. which had been derived from the fluid catalytic cracking, using a synthetic catalyst, of a gas oil. Some tests were made utilizing a mixture of the catalytically cracked SHN and a gasoline base stock which consisted of a mixture of virgin and thermally cracked naphthas.

The tests were carried out in a small continuous unit. This unit consisted of two l-gallon vessels connected in series; each vessel was preceded by an orifice-type mixer. These vessels operated as settlers and the lower aqueous phase was withdrawn from the settler and circulated to the naphtha line ahead of the mixer by means of a pump.

The temperature maintained in the system was con trolled by means of steam heaters in the aqueous solution circulating lines. In all of the tests each vessel was charged with 2.5 liters of aqueous caustic solution con- 0 taining 2l weight percent of NaOH.

The unit was so arranged that air could be injected into the naphtha line between the irst vessel and the second mixer. The amount of air injected was regulated by means of a rotameter.

The catalytically cracked SHN was held in SO-gallon drums which had been swept out with nitrogen. The catalytic SHN was obtained directly from the fractionating tower prior to exposure to the atmosphere.

When inhibitor was to be added directly to the rawnaphtha a measured solution was added to the SHN in the drum and the contents thoroughly agitated with nitrogen.

When it was desired to add inhibitor to the naphtha subsequent to treatment of the naphtha with aqueous caustic solution, a measured amount of inhibitor was added to the eluent from the rst vessel of the sweetening unit.

In all tests the phenylene diamine inhibitor used was N,Ndisecondarybutylp-phenylene diamine.

In these tests the initial existent gum was determined by ASTM Method D381-49. The potential gum was determined by the accelerated method ASTM Method D873-49; the period of the test was 4 hours. a

Tests not reported herein showed that impracticably long reaction times were needed when the operating temperature was lower than 160 F. for addition of inhibitor to the caustic-treated naphtha. All tests reported below wherein the inhibitor was added to the caustic-treated naphtha, i. e., introduced into the naphtha line subsequent to vessel 1, were carried out at 160 F. and a doctor sweet product was obtained in all tests. In Table I below are reported the results of sweetening the catalytically cracked SHN using various amounts of the defined inhibitor when the inhibitor was added subsequent to aqueous caustic treating of raw-naphtha.

Table I Potentlal Gum, 4Hr.

Exlstent Gum Inhibitor, Lbs.fM Bbls.

Test In Table II below are presented data on tests' carried out as for the data in Table I except that the feed to the process consisted of the catalytic SHN and the mixed base in equal amounts. The inhibitor usage is based on total naphtha charged so that for the usage based on SI-IN alone the gures should be multiplied by 2.

Table II Inhibitor, Potential Exlstent Test Lbs./M Gum, 4-Hr. Gum

Bbls.

of the catalytically-crackedSHN.

miramos F.' `Theresa fin-al1 me inv rableimaonsisiei `Table .Ill

Inhibitor,` `Potential Existent Test .Lbs/M fGnmA-Hr. .Gum

Bbls.

In Table IV below the equal volume mixture of SHN and mixed base was used as the feed and the sweetening procedure followed that used in the tests of Table III.

The tests set out in Tables I and III show a remarkable difference in potential gum content of the sweet naphthas produced by the two methods of operation. This decrease from a gum content of between about 450 and 500 mg./ 100 ml. of naphtha to a potential gum content on the order of 20 mg./ 100 ml. is truly remarkable when it is considered that this improvement is obtained as a result of a shift in point of introduction of the inhibitor and nothing else. The data indicate that a very large improvement in gum content is obtainable with the use of more than 1 lb./M bbls. of inhibitor by the introduction of said inhibitor prior to the initial aqueous caustic contacting.

The data set out in Tables II and IV show that the remarkable improvement of gum stability also occurs when the cracked naphtha is diluted with as much as an equal volume of a relatively stable naphtha. Thus the process of this invention may be used not only on raw naphthas consisting entirely of cracked materials, but may also be used wherein the cracked naphtha is in admixture with virgin naphthas.

The annexed figure which forms a part of this specification sets out in schematic form an illustrative embodiment of the process of this invention.' It is to be understood that the illustrative embodiment does not include numerous items of equipment and process details which may be readily added thereto by those skilled in this art.

In the gure the naphtha fraction from the uid catalytic cracking, using a synthetic catalyst, of a mixture of gas oils is passed directly from the cracking operation not shown by way of line 11 into fractionator 12. In fractionator 12 the total naphtha is stabilized. An overhead fraction consisting of propane, butane and some pentane is withdrawn from fractionator 12 by way of line 13 and is passed to further processing not shown. The stabilized cracked naphtha which has a mercaptan number of about is withdrawn from the bottom of fractionator 12 by way of line 14.

Four pounds of N,Ndisecondarybutylp-phenylene diamine inhibitor per 1000 barrels of naphtha in line 14 from source 16 is introduced by way of line 17 into line 18 where it meets the cracked, sour raw-naphtha from line 14.

The raw-naphtha-inhibitor mixture is passed from line 18 into mixer 19. Aqueous sodium hydroxide solution from line 21 is introduced into mixer 19. Mixer 19 may be either a knothole-type mixer or a mechanically stirred mixer. In mixer 19 the aqueous caustic solution and the mixture are intimately contacted. The contents of mixer 19 are passed by way of line 22 into settler 23.

Settler 23 may be any form of vessel for permitting :the gravitytseparationrdf:twoaimmiscible phases.` Azlower Vlayer V-of .aqueous :caustic solution is withdrawn. `'trom settlerL 2'31by way of line 2L4;

`This aqueous caustic solution :ordinarily -,is vrecycled to mixer `=19 :by .way of -valved `line .26, heat exchanger 27 and line 21. In `heat exchanger 27 .the :temperature Aof -`the lcaustic solution is raised to a temperature `isuch .that thezzcontents lof mixer 19 are at about-90 F. The aqueous causticisolutionfis presentin an amount of about -20 :volume :percent based on .naphtha charged from line 1.4.

The naphtha from line 14 contains hydrogen sulfide, cresols and mercaptans. These materials react with the sodium hydroxide and decrease the effective strength of the aqueous caustic solution. Periodically when the aqueous caustic solution has reached a concentration of below about l0 weight percent free sodium hydroxide, a portion of the aqueous caustic solution from line 24 is withdrawn from the system by way of valved line 29.

Makeup aqueous caustic solution is introduced into the system from source 31 by way of valved lines 32 and 26. In this illustration the aqueous caustic solution contains 25 weight percent of sodium hydroxide.

The extracted naphtha is passed from settler 23 by way of line 34. Commercial grade cylinder oxygen from source 36 is introduced by way of line 37 into line 34. Herein there is utilized about 6 s. c. f. per pound of mercaptan sulfur present in the extracted naphtha.

The naphtha-oxygen stream is passed by way of line 38 into mixer 39. Aqueous caustic solution from line 41 is introduced into mixer 39. Mixer 39 is similar in construction to mixer 19. The contents of mixer 39 are passed by way of line 42 into settler 43.

Settler 43 is similar in construction to settler 23. The lower aqueous caustic layer is withdrawn from settler 43 by way of line 46 and is recycled to the process by Way of valved line 47, heat exchanger 48 and line 41. In heat exchanger 48 the circulating aqueous caustic solution is raised to a temperature such that the contents of mixer 39 are maintained at a temperature of about 90 F.

The aqueous caustic solution withdrawn from settler 43 eventually becomes contaminated with reaction products. Periodically a portion of the circulating stream may be withdrawn from the system by way of line 46 and valved line 51. This contaminated aqueous caustic solution contains a considerable amount of free-caustic. Therefore a portion of the aqueous caustic solution may be passed by way of line 46 and valved line 52 to line 24 for use in the rst aqueous caustic treating zone.

Fresh aqueous caustic solution from source 56 is introduced by way of line 57 into line 47. This aqueous caustic solution contains 25% of sodium hydroxide. Suicient makeup solution is added to maintain in the system about 20 volume percent of aqueous solution based on naphtha introduced from line 38. A sweet product naphtha is withdrawn from settler 43 by way of line 61 and is passed to storage not shown.

Thus having described the invention, what is claimed is:

l. A process of sweetening a cracked, sour rawnaphtha, which process comprises (l) adding to said raw naphtha, substantially immediately after said raw naphthas production from the liquid product of the prior cracking operation, a phenylene diamine inhibitor, in an amount between about 4 and 10 pounds per 1000 barrels of said naphtha, (2) contacting said mixture, at a temperature between about and 100 F. with an aqueous caustic solution containing between about 15 and 30 weight percent of alkali metal hydroxide, in an amount between about 5 and 100 volume percent based on said naphtha, (3) separating a sour, extracted naphtha phase from an aqueous solution layer, (4) introducing into said extracted naphtha free-oxygen in an amount between about and 250% @of the stoichiometric equivalent of mercaptan sulfur :present insaid extracted naphtha, (5) contacting the loxygen-naphtha mixture with an aqueous lcaustic soLution containing between about 15 and 3Q weight percent of alkali metal hydroxide, in an amount between .about 54 and 100 volume percent based on extracted naphtha, ata temperature between about 80 and 100 F., for. a time suicient to substantially sweeten `said extracted naphtha, and (6) separating an aqueousvcaustic layer Vfrom a substantially sweet naphtha phase.

, 2. The process of claim 1 wherein said raw-naphtha has been derived from a catalytic cracking process.

References Cited in the file of this patent UNITED STATES PATENTS Tom et al. Apr. 3, Browder May 8, Chenicek et al Nov. 4,

Johnstone Apr. 7, 

1. A PROCESS OF SWEETENING A CRACKED, SOUR RAW NAPHTHA, WHICH PROCESS COMPRISES (1) ADDING TO SAID RAW NAPTHA, SUBSTANTIALLY IMMEDIATELY AFTER SAID RAW NAPTHA''S PRODUCTION FROM THE LIQUID PRODUCT OF THE PRIOR CRACKING OPERATION, A PHENYLENE DIAMINE INHIBITOR, IN AN AMOUNT BETWEEN ABOUT 4 AND 10 POUNDS PER 1000 BARRELS OF SAID NAPTHA, (2) CONTACTING SAID MIXTURE, AT A TEMPERATURE BETWEEN ABOUT 80* AND 100* F. WITH AN AQUEOUS CAUSTIC SOLUTION CONTAINING BETWEEN ABOUT 15 AND 30 WEIGHT PERCENT OF ALKALI METAL HYDROXIDE, IN AN AMOUNT BETWEEN ABOUT 5 AND 100 VOLUME PERCENT BASED ON SAID NAPHTHA, (3) SEPARATING A SOUR, EXTRACTED NAPHTHA PHASE FROM AN AQUEOUS SOLUTION LAYER, (4) INTRODUCING INTO SAID EXTRACTED NAPTHA FREE-OXYGEN IN AN AMOUNT BETWEEN ABOUT 150 AND 250% OF THE STOICHIOMETRIC EQUIVALENT OF MERCAPTAN SULFUR PRESENT IN SAID EXTRACTED NAPTHA, (5) CONTACTING THE OXYGEN-NAPTHA MIXTURE WITH AN AQUEOUS CAUSTIC SOLUTION CONTAINING BETWEEN ABOUT 15 AND 30 WEIGHT PERCENT OF ALKALI METAL HYDROXIDE, IN AN AMOUNT BETWEEN ABOUT 5 AND 100 VOLUME PERCENT BASED ON EXTRACTED NAPHTHA, AT A TEMPERATURE BETWEEN ABOUT 80* AND 100* F., FOR A TIME SUFFICIENT TO SUBSTANTIALLY SWEETEN SAID EXTRACTED NAPTHA, AND (6) SEPARATING AN AQUEOUS CAUSTIC LAYER FROM A SUBSTANTIALLY SWEET NAPTHA PHASE. 